This invention relates to a multiple riser catalytic cracking operation in which mobile hydrogen species and/or carbon-hydrogen molecular fragments are employed to increase conversion of a hydrogen-deficient heavy hydrocarbon feed, e.g., a resid, to useful products contributing to gasoline boiling range material.
In known and conventional fluidized catalytic cracking processes, a relatively heavy hydrocarbon feedstock, e.g., a gas oil, admixed with a suitable cracking catalyst to provide a fluidized suspension is cracked in an elongated reactor, or riser, at elevated temperature to provide a mixture of lighter hydrocarbon products. The gasiform reaction products and spent catalyst are discharged from the riser into a separator, e.g., a cyclone unit, located within the upper section of an enclosed stripping vessel, or stripper, with the reaction products being conveyed to a product recovery zone and the spent catalyst entering a dense catalyst bed within the lower section of the stripper. In order to remove entrained hydrocarbon product from the spent catalyst prior to conveying the latter to a catalyst regenerator unit, an inert stripping gas, e.g., steam, is passed through the catalyst where it desorbs such hydrocarbons conveying them to the product recovery zone. The fluidizable catalyst is continuously circulated between the riser and the regenerator and serves to transfer heat from the latter to the former thereby supplying the thermal needs of the cracking reaction which is endothermic.
Particular examples of such catalytic cracking processes are disclosed in U.S. Pat. Nos. 3,617,497, 3,894,932, 4,309,279 and 4,368,114 (single risers) and U.S. Pat. Nos. 3,748,251, 3,849,291, 3,894,931, 3,894,933, 3,894,934, 3,894,935, 3,926,778, 3,928,172, 3,974,062 and 4,116,814 (multiple risers). Several of these processes employ a mixed catalyst system with each component of the system possessing different catalytic properties and functions. For example, in the dual riser hydrocarbon conversion process described in aforesaid U.S. Pat. No. 3,894,934, a heavy hydrocarbon first feed, e.g., a gas oil, is cracked principally as a result of contact with a large pore crystalline silicate zeolite cracking catalyst, e.g., zeolite Y, to provide lighter products. Spent catalyst is separated from the product stream and enters the dense fluid catalyst bed in the lower section of the stripping vessel. A C.sub.3-4 olefin-rich second feed, meanwhile, undergoes coversion to cyclic and/or alkylaromatic hydrocarbons in a second riser, principally as a result of contact with a shape selective medium pore crystalline silicate zeolite, e.g., zeolite ZSM-5. Spent catalyst recovered from the product stream of the second riser similarly enters the dense catalyst bed within the stripper vessel. U.S. Pat. No. 3,894,934 also features the optional introduction of a C.sub.3.sup.- containing hydrocarbon third feed along with an aromatic-rich charge into the dense fluid bed of spent catalyst above the level of introduction of the stripping gas to promote the formation of alkyl aromatics therein. As desired, the third feed may be light gases obtained from a fluid cracking light ends recovery unit, virgin straight run naphtha, catalytically cracked naphtha, thermal naphtha, natural gas constituents, natural gasoline, reformates, a gas oil, or a residual oil of high coke-producing characteristics.
U.S. Pat. No. 3,894,935 describes a dual riser fluid catalytic cracking process in which a gas oil is catalytically cracked in a first riser in the presence of a faujasite-type zeolite such as zeolite Y to provide gasoline boiling-range material and a C.sub.3-4 -rich hydrocarbon fraction or isobutylene is converted in a second riser in the presence of hot regenerated catalyst or catalyst cascaded thereto from the first riser to provide aromatics, alkyl aromatics and low boiling gaseous material.
In fluidized catalytic cracking operations employing mixtures of large and medium pore size crystalline silicate zeolite catalysts where catalyst separated from the product effluent is conveyed to a stripper and from there to a catalyst regenerating zone, regardless of the nature of the catalyst introduction at start-up, once steady-state operation has been achieved, the two types of catalyst will become fairly uniformly mixed and will circulate throughout the system at or about the same rate. This arrangement is subject to a significant disadvantage. While the large pore zeolite cracking catalyst cokes up relatively quickly and must therefore be regenerated at frequent intervals, this is not the case with the medium pore zeolites which can maintain their catalytic activity over many more cycles of operation. However, since the large and medium pore zeolites are in intimate admixture, heretofore there has been no practical means of conveying only the large pore zeolite to the catalyst regenerator unit or, what amounts to the same thing, keeping the medium pore zeolite, or at least most of it, on the average out of the regenerator.
Thus, a principal disadvantage resulting from the use of mixed catalyst systems in known fluidized catalytic cracking operations is owing to the fact that the medium pore zeolite component is subjected to the harsh hydrothermal conditions of the catalyst regenerator unit even though it does not require regeneration anywhere near the rate at which the large pore zeolite cracking component must be regenerated. The medium pore zeolite is therefore needlessly subjected to hydrothermal deactivation at a much greater rate than is necessary for it to function.
U.S. Pat. No. 4,116,814 describes a multiple riser fluidized catalytic cracking operation utilizing a mixture of large and medium pore crystalline zeolite catalysts which differ in particle size and/or density as to facilitate their separation in a common catalyst regeneration unit. There is, however, no hint in this patent of preventing the transfer or reducing the rate of circulation of medium pore crystalline zeolite to and through the catalyst regeneration unit.
U.S. Pat. No. 4,287,088 describes a process and system for the segregation of used contaminated catalyst into fractions according to particle density differences. No mention is made of mixed catalyst systems.
It is known to upgrade hydrogen-deficient heavy hydrocarbon feedstocks such as gas oils, resid, syncrudes, etc., to more valuable products by thermal and catalytic cracking operations in admixture with a hydrogen donor diluent material. The hydrogen donor diluent is defined as a material, which releases hydrogen to a hydrogen-deficient oil in a thermal or catalytic cracking operation.
One advantage of a hydrogen donor diluent operation is that it can be relied upon to convert heavy oils or hydrogen-deficient oils at relatively high conversions in the presence of catalytic agents with reduced coke formation. Coke as formed during the cracking operation is usually a hydrocarbonaceous material sometimes referred to as a polymer of highly condensed, hydrogen-poor hydrocarbons.
Catalytic cracking systems in current operation, e.g., those referred to above, have taken advantage of new catalyst developments, that is, the use of large pore crystalline silicate zeolite cracking catalysts in preference to the earlier used amorphous silica-alumina cracking catalysts. These new crystalline zeolite cracking catalysts, e.g., zeolites X and Y, are generally regarded as low coke producing catalysts. Thus, as the level of coke deposits has been reduced through the use of crystalline zeolite cracking catalysts, it has been equally important to concentrate on recovering the maximum amount of heat available through the burning of deposited coke in the regenerator. However, when operating a catalytic cracking process within optimum conditions provided by the crystalline zeolite conversion catalysts, the petroleum refiner is still faced with operating a hydrogen-deficient process which does not permit the most optimistic recovery of desired products.
In accordance with the hydrocarbon conversion process described in U.S. Pat. No. 4,035,285, a low molecular weight carbon-hydrogen contributing material and a high molecular weight feedstock, e.g., a gas oil, are combined and reacted in the presence of one or more crystalline silicate zeolite catalysts, e.g., zeolite Y, in admixture with ZSM-5, the resulting cracking and carbon-hydrogen additive reactions producing products of improved quality and superior to those formed in the absence of the low molecular weight carbon-hydrogen contributing material. Advantages of the process include improved crackability of heavy feedstocks, increased gasoline yield and/or higher gasoline quality (including octane and volatility), and fuel oil fractions of improved yield and/or burning quality and lower levels of potentially polluting impurities such as sulfur and nitrogen. In addition, the need for high pressure hydrotreaters and hydrocrackers using relatively expensive molecular hydrogen-rich gas can be eliminated or the severity requirements of the operation greatly decreased.
A similar process in which full range crude oils and naphtha are catalytically cracked in the presence of such low molecular weight carbon-hydrogen contributing material and zeolites in separate risers of a multiple riser catalytic cracking unit is described in U.S. Pat. No. 3,974,062 referred to supra.